Wireless communication systems that were providing voice-based services have evolved to broadband wireless communication systems that are capable of providing packet data services based on high quality and high speed, such as: Long Term Evolution (LTE), High Speed Packet Access (HSPA) defined in 3GPP; Ultra Mobile Broadband (UMB), High Rate Packet Data (HRPD) defined 3GPP2; the communication standard IEEE 802.16e; etc.
The LTE system, as a typical example of the broadband wireless communication systems, employs Orthogonal Frequency Division Multiplexing (OFDM) in the downlink and Single Carrier-Frequency Division Multiple Access (SC-FDMA) in the uplink. The Multiple Access performs allocation and management of time-frequency resources to carry data and control information according to users, so as not to overlap each other, i.e., so as to achieve orthogonality between them, thereby distinguishing data or control information between respective users.
The LTE system employs a Hybrid Automatic Repeat reQuest (HARQ) scheme for retransmitting data, which has failed in decoding in the initial transmission, via the physical layer. HARQ is a scheme that allows a receiver to transmit, when not correctly decoding data from a transmitter, information (NACK) indicating the decoding failure to the transmitter so that the transmitter can perform re-transmission of the data from the physical layer. The receiver combines the data re-transmitted from the transmitter with the existing data for which decoding has failed, thereby increasing the capability of data reception. When correctly decoding data, the receiver transmits information (ACK) indicating the success of decoding to the transmitter so that the transmitter can perform transmission of new data.
In broadband wireless communication systems, one of the important factors in providing high transmission rate wireless data services is the ability to support scalable bandwidths. For example, LTE systems are capable of supporting various bandwidths, such as 20/15/10/5/3/1.4 MHz, etc. Therefore, service providers are capable of selecting a particular one of the various bandwidths and providing services via the bandwidth. There are various types of user equipment (UE) devices that are capable of supporting bandwidths from a minimum of 1.4 MHz to a maximum of 20 MHz.
LTE-Advanced (LTE-A) systems, aiming to provide a level of service for IMT-Advanced requirements, are capable of providing services in broadband up to the maximum of 100 MHz, by aggregating LTE carriers. LTE-A systems require to be wider than that of LTE systems for high-speed data transmission. In addition, LTE-A systems need to allow for the backward compatibility with LTE user equipment (UE) devices, so that the LTE UE devices can access the services of the LTE-A systems. To do this, LTE-A systems divide the whole system bandwidth into sub-bands or component carriers (CC), through which LTE UE devices are capable of transmission or reception, and aggregate part of the component carriers. LTE-A systems are capable of creating data according to respective component carriers and performing transmission of the created data. LTE-A systems are capable of high speed data transmission in the transmission/reception processes of the legacy LTE systems used according to the respective component carriers.
FIG. 1 shows a schematic diagram of a conventional LTE-A system capable of carrier aggregation. As shown in FIG. 1, eNB102 supports the aggregation of two component carriers, CC#1 and CC#2. CC#1 has a frequency f1 and CC#2 has a frequency f2 that differs from f1. CC#1 and CC#2 are equipped with the same base station (eNB102). The eNB102 provides coverages 104 and 106 corresponding to the component carriers CC#1 and CC#2 respectively. The LTE-A system capable of carrier aggregation performs transmission of data and transmission of control information related to the data transmission according to component carriers respectively. The configuration shown in FIG. 1 may be applied to the aggregation of uplink carriers the same way as the aggregation of downlink carriers.
The carrier aggregation system divides component carriers into Primary Cell (Pcell) and Secondary Cell (Scell) to manage them. Pcell refers to a cell that provides the basic radio resources to UE and serves as a standard cell allowing UE to perform operations such as the initial access, a handover, etc. Pcell includes a downlink primary frequency or Primary Component Carrier (PCC) and an uplink primary frequency. Scell refers to a cell that provides additional radio resources to UE along with Pcell. Scell includes a downlink secondary frequency or Secondary Component Carrier (SCC) and an uplink secondary frequency. In the present disclosure, unless otherwise indicated, the terms ‘cell’ and ‘component carrier’ will be used interchangeably with each other.
Conventional LTE-A systems capable of carrier aggregation had a restriction that they need to perform carrier aggregation within the same eNB.
In recent years, carrier aggregation between different eNBs has been discussed. When performing carrier aggregation between different eNBs, there is a need to clearly define a method for UE to request scheduling for the uplink data transmission.